I now own two Hartke amps...an HA-5000 and an HA-4000. If the amps didn't have brand names on them, you'd swear (IMO) that they were built by different companies as there is little family resemblance. The HA-5000 has an aluminum front panel, blendable solid state and tube preamps, a graphic EQ, on-board compression, active bass and treble controls (Hi/Lo contour), smoothly rotating controls, toroidal power transformer and a fixed speed fan. The HA-4000 has a plastic front panel, 100% solid state construction, variable contour control in the mids, four band semi-parametric EQ, a low harmonic enhancer, detent-like controls, mute button, tuner out, E-I construction power transformer (IIRC) variable speed cooling fan, etc. In other words, they couldn't be more different. Anyone know why?